The present invention relates to a material that is useful in multiple applications including insulation applications for garments, containers, pipes, electronic devices and the like. Among other things, the material of the present invention comprising aerogel particles and polytetrafluoroethylene (PTFE), is formable, having low particle shedding and low thermal conductivity. Composites made from the material may be flexed, stretched, and twisted, with little or no shedding of aerogel particles or loss of conductive properties.
Use of aerogels for thermal insulation and the low thermal conductivity of aerogels is well known. Favorable thermally conductive properties result from the very high porosity of aerogel which is greater than about 95%, and the small pore size of aerogel material which is less than the size of the mean free path of air molecules at atmospheric pressure, or less than about 100 nm. Because of the small pore size, the mobility of air molecules within the material is restricted, and the effectiveness of air in conducting heat is reduced, resulting in low thermal conductivity. Under atmospheric conditions air has a thermal conductivity of about 25 mW/m K (milliwatt per meter Kelvin). Insulation having larger pore sizes, such as foam, batting, wool, and other common thermally insulating materials, has a thermal conductivity of about 40 mW/m K, which is higher than that of air due to the contribution of radiation and solid conduction. Aerogel powders and beads are known to have a thermal conductivity of about 9 to 20 mW/m K. However, such highly porous and low density material is not useful for many applications in the form of a powder due to the extensive dusting which makes installation, handling, forming and shaping particularly difficult, and further raises safety issues.
Traditional methods of making aerogels usually include extraction with supercritical fluids. The methods often include the steps of pouring an aerogel precursor liquid into a mold, drying the aerogel liquid to form a highly porous gel structure with a variety of liquid exchanges, and using supercritical fluid extraction to form an aerogel monolith. Processes, such as those using supercritical fluid extraction, are very time consuming and expensive. Further, the structures produced are rigid and have low mechanical strength and have limited ability to be further molded or formed into desired shapes after the aerogel material is formed. These materials often crack or shatter upon flexing and are known for shedding or “dusting” of fine aerogel particles.
In an attempt to increase the flexibility and strength of aerogel material, Stepanian et al. U.S. Patent Publication 2002/0094426 teach aerogel materials combined with a reinforcing structure, specifically a lofty fibrous batting. Preferably, the aerogel is reinforced by a fibrous batting structure in combination with randomly oriented microfibers and/or conductive layers. To form the aerogel sheet, an aerogel-forming precursor liquid is poured into the batting and supercritically dried to form an aerogel. It is taught that the resulting reinforced aerogel structure is drapable, less prone to shattering upon flexing and less prone to shedding of fine aerogel particles. However, applications for such materials are limited due to a lack of moldability and formability of these structures, as well as the costs associated with supercritical extraction steps.
To overcome the brittleness often associated with reinforced aerogels, U.S. Pat. No. 5,786,059, to Frank et al. teaches gluing aerogel powders together to form a continuous product. Specifically, an aerogel composite material having a layer of fiber web and aerogel particles is preferably formed as a mat or panel. The fiber web comprises a bicomponent fiber material of two firmly interconnected polymers having lower and higher temperature melting regions into which aerogel particles are sprinkled. Upon heating to the lower melt temperature, the fibers of the web are bonded to each other as well as to the aerogel particles. The resulting composites are relatively stiff structures, and upon the application of mechanical stress, granules break or become detached from the fiber so that aerogel fragments may fall out from the web.
Smith et al., in U.S. Pat. No. 6,172,120, disclose a process for the manufacture of aerogels wherein aerogels are formed as powders instead of monolithic blocks or sheets. The manufacturing process has the advantage of aerogel formation without the step of supercritical fluid extraction. However, in the form of a powder, aerogel is not useful for many applications due to high dusting and lack of formability.
There is a need for an insulating material that overcomes problems inherent in aerogel powders and composites, such as the lack of formability of aerogel powder and the lack of flexibility of composites, as well as the shedding or dusting of aerogel particles upon application of mechanical stress. There is a need for low conductivity, low shedding insulating materials that can be molded or formed into any desired shape subsequent to the formation of the insulating material composition, and which, therefore, can be easily suited to a wide variety of applications. Further, insulating materials which may be prepared without the high processing cost often associated with aerogel insulation would be greatly desired.